docs: def to_dim_times_two(self, bounds):
          """return boundaries in format ``[[lb0, ub0], [lb1, ub1], ...]``,
          as used by ``BoxConstraints...`` class.
  
          """
          if not bounds:
              b = [[None, None]]
          else:
              l = [None, None]  # figure out lenths
              for i in [0, 1]:
                  try:
                      l[i] = len(bounds[i])
                  except TypeError:
                      bounds[i] = [bounds[i]]
                      l[i] = 1
              b = []  # bounds in different format
              try:
                  for i in xrange(max(l)):
                      b.append([bounds[0][i] if i < l[0] else None,
                                bounds[1][i] if i < l[1] else None])
              except (TypeError, IndexError):
                  print("boundaries must be provided in the form " +
                        "[scalar_of_vector, scalar_or_vector]")
                  raise
          return b